In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge; or known to be relevant to an attempt to solve any problem with which this specification is concerned.
Polyethylene terephthalate (PET) is a thermoplastic polyester formed by the reaction of ethane-1,2-diol (also known as ethanediol, 1,2-dihydroxyethane, monoethylene glycol or MEG) and purified terephthalic acid (PTA) by direct esterification to form bis-(hydroxyethyl) terephthalate ester (“BHET”) which is then polymerised by catalysed ester exchange to useful polymers.
Traditionally, PET has been used extensively because it can be formed into an oriented film or fibre, has high tenacity, good electrical resistance, low moisture absorption, and a melting point greater than 245° C. PET has been found to be particularly valuable for the manufacture of bottles for containing carbonated drinks where bottles must have good tensile strength, exhibit superior mechanical properties, contain pressure, and have a low mass convenient for transport and use. Such PET bottles are also extensively used for non-carbonated drinks.
The extensive use of PET bottles has given rise to the need to recycle PET bottles after their contents are consumed. Similarly, there is a need to recycle post-use PET film, PET textiles and apparel products, and the in-process PET waste arising from the manufacture of all PET products. Today, PET is mainly recycled using either of two distinct methods, namely mechanical and chemical.
In the mechanical process, PET waste (which includes post-consumer PET) is cut and washed to obtain PET flakes. These flakes are then converted to various end products using conventional polyester processes. However, such flakes are frequently contaminated leading to products having a quality which is not comparable with product made from the high purity virgin petrochemical feedstocks typically used for making polyester.
Conventionally, chemical recycling of PET relies on depolymerising the PET waste by treating it with a significant concentration of glycol at high temperature. This glycolysis yields a product consisting primarily of esters of varying molecular weights. The mixture of esters generally includes BHET, dimers, trimers, tetramers and higher oligomers. Such esters from conventional chemical recycling processes cannot be stored as a liquid for prolonged periods because elevated temperatures (above 200° C.) result in degradation of the mixtures of the esters and formation of unfavourable by-products which are detrimental to the desired quality and consistency of the end products. As a result, the chemical recycler of waste PET at a given plant generally processes the recovered esters immediately for a narrow single application or final product form.
For example, GB 610,136 discloses a process for obtaining BHET and higher oligomers through the de-polymerisation of polyester scraps by using ethylene glycol and then obtaining reconstituted polyester by polymerising the BHET and higher oligomers immediately. This is done because the BHET and higher oligomers, thus obtained, is unstable when stored at high temperature and degrades.
In another example, U.S. Pat. No. 4,609,680 discloses a process for depolymerisation of polyester scrap wherein PET scrap is depolymerised to bis-hydroxyethyl terephthalate (BHET) and/or its oligomers by supplying the scrap, together with ethylene glycol, to a reactor containing molten BHET from which a part of the product in the reactor is removed to a polymerising reactor.
Current chemical PET recycling processes thus preclude the recovered ester from a single recycling plant from being used across a wider range of downstream product applications. This increases the commercial risk and burden on capital investment for a recycler using such chemical recycling processes to install such downstream conversion facilities within the same complex, tailored narrowly to a particular end product and market.
Hence, there is need for a process wherein the esters produced by the glycolysis of PET waste can be stored for use as and when desired.